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SYSLOGD(8) System Manager's Manual SYSLOGD(8)

syslogd — Apple System Log server

syslogd [-d] [-D] [-m mark_interval] [-l lib_path] [-db_max size] [-utmp_ttl time] [-mps_limit quota] [-dup_delay time] [-module_name {0|1}]

The syslogd server receives and processes log messages. Several modules receive input messages through various channels, including UNIX domain sockets associated with the syslog(3), asl(3), and kernel printf APIs, and optionally on a UDP socket from network clients.

The Apple System Log facility comprises the asl(3) API, a new syslogd server, the syslog(1) command-line utility, and a data store file manager, aslmanager(8). The system supports structured and extensible messages, permitting advanced message browsing and management through search APIs and other components of the Apple system log facility.

Log messages are retained in a data store, subject to automatic archival, and input filtering as described below, to simplify the task of locating log messages and to facilitate browsing and searching. The data store is intended to become a replacement for the numerous log files that are currently found in various locations on the system. Those files will be phased out in future versions of Mac OS.

The following options are recognized:

Run syslogd in debugging mode. The server stays attached to the controlling terminal and prints debugging messages.
Start as a daemon. This option forces syslogd to fork and have the child process become a daemon. Since syslogd is started by launchd, this is not normally required.
Set the number of minutes between “mark” messages. Mark messages are normally disabled. If -m is specified with no arguments, mark messages will be written every 20 minutes. The “mark” facility is disabled if the setting is zero minutes.
Sets the size limit in bytes for individual files in the data store. The default value for -db_max is 25600000 bytes. Files are closed upon reaching the maximum size, and a new file is opened for subsequent messages.
Sets the time-to-live in seconds for messages used by the utmp(5), wtmp(5), and lastlog(5) subsystems. The default is 31622400 seconds (approximately 1 year). Note that if archival is enabled (see the aslmanager(8) manual), these messages will be copied to an archive after the regular time-to-live interval, but will persist in the data store until their own expiry time.
Sets the kernel quota for messages per second allowed by syslogd. Any messages in excess of the quota limit from any process are ignored. An error message is logged stating that the kernel message quota has been exceeded, and that remaining messages for the current second will be discarded. The default limit is 500 messages per second per process. A value of 0 turns off the quota mechanism.

Note that this setting only limits the number of kernel messages that will be saved by syslogd. User processes are limited to 36000 messages per hour. The limit for a user process is not enforced if a remote-control ASL filter is in place for the process.

Sets the time to delay for coalescing duplicate message in log files. If a process logs multiple messages with the same text, syslogd will wait for the specified period of time to coalesce duplicates. If identical messages arrive during this interval, syslogd will print a message of the form:

May 7 12:34:56: --- last message repeated 17 times ---

The default delay time is 30 seconds. Setting the value to 0 disables the coalescing mechanism.

The remaining options of the form -module_name {0|1} may be used to disable (0) or enable (1) the action of several of syslogd 's internal modules.

The “asl_in” module receives log messages on the UNIX domain socket associated with the asl(3) API. The module may be disabled using -asl_in 0. The module is normally enabled.
The “asl_action” module examines the stream of received log messages and acts upon them according to the rules specified in the file /etc/asl.conf. See asl.conf(5) for details.
The “klog_in” module receives log messages on the UNIX domain socket associated with the kernel logging API. The module may be disabled using -klog_in 0. The module is normally enabled.
The “bsd_out” module examines the stream of received log messages and acts upon them according to the rules specified in the file /etc/syslog.conf. See syslog.conf(5) for details. This module exists for backward compatibility with previous syslogd implementations. Apple encourages use of the syslog(1) and asl(3) search APIs over the use of the log files that are specified in the /etc/syslog.conf file. Future versions of Mac OS will move functions that are currently handled by the “bsd_out” module to the “asl_action” module.
The “udp_in” module receives log messages on the UDP socket associated with the Internet syslog message protocol.

This module is normally enabled, but is inactive. The actual UDP sockets are managed by launchd, and configured in the syslogd configuration file /System/Library/LaunchDaemons/com.apple.syslogd.plist. In the default configuration, launchd does not open any sockets for the “syslog” UDP service, so no sockets are provided to the “udp_in” module. If no sockets are provided, the module remains inactive. A socket may be specified by adding the following entry to the “Sockets” dictionary in the com.apple.syslogd.plist file.

<key>NetworkListener</key>
<dict>
<key>SockServiceName</key>
<string>syslog</string>
<key>SockType</key>
<string>dgram</string>
</dict>

The module may be specifically disabled using the -udp_in 0 option.

syslogd reinitializes in response to a HUP signal.

syslogd periodically invokes the aslmanager utility, which manages files in the ASL data store. Files are removed or optionally copied to an archival directory after a (default) 2 day time-to-live. See the aslmanager(8) manual for details. syslogd invokes aslmanager shortly after it starts up, at midnight local time if it is running, and any time that a data store file reaches the -db_max size limit.

Messages saved in the ASL message store are written to files in /var/log/asl. The message files are given read access controls corresponding to the read UID and GID specified in the messages themselves. Read access UID and GID settings may be attached to messages using the asl(3) library by setting a value for the "ReadUID" and/or "ReadGID" message keys. The file permissions prevent access-controlled messages from being read by unauthorized users.

Although clients are generally free to use any value for the "Facility" message key, only processes running with UID 0 may log messages with a facility value of "com.apple.system", or with a value that has "com.apple.system" as a prefix. Messages logged by non UID 0 processes that use "com.apple.system" as a facility value or prefix will be saved with the facility value "user".

/etc/syslog.conf
bsd_out module configuration file
/etc/asl.conf
asl_action module configuration file
/var/run/syslog.pid
process ID file
/dev/klog
kernel log device
/var/log/asl
data store directory
/var/log/asl.archive
default archive directory
/System/Library/LaunchDaemons/com.apple.syslogd.plist
launchd configuration file for syslogd

logger(1), syslog(1), asl(3), syslog(3), asl.conf(5), syslog.conf(5)

The syslogd utility appeared in 4.3BSD.

The Apple System Log facility was introduced in Mac OS X 10.4.

October 18, 2004 macOS